1. Field of the Invention
The present invention relates to a radio transmission apparatus and radio reception apparatus in a digital radio communication.
2. Description of the Related Art
Digital radio communications always have a possibility that a received electric field strength of a signal becomes weak due to geographical conditions because a space is used as a transmission medium. Therefore, when the received electric field strength of the signal is weak, an error rate of data is increased and the data quality deteriorates. An example of measures to cope with the above situation is a technique using a polarization modulation as described in Japanese Laid Open Patent Publication HEI4-276926.
This technique uses a modulation system that represents “1” and “0” of data with polarizations. Specifically in the technique, a transmission apparatus and reception apparatus each is provided with two antennas with different directivities from each other, and the apparatuses communicate signals using antennas with the same directivity. For example, when the transmission apparatus and reception apparatus each is provided with two antennas with different directivities from each other (dextrorotatory and levorotatory), a signal transmitted from a dextro-antenna of the transmission apparatus can be received only at the dextro-antenna of the reception apparatus, and a signal transmitted from a levo-antenna of the transmission apparatus can be received only at the levo-antenna of the reception apparatus.
Accordingly, for example, by determining in advance that transmission data from a dextro-antenna represents “1”, and that transmission data from a levo-antenna represents “0”, the reception apparatus Is capable of deciding “0” or “0” of data by recognizing that the data is transmitted from either antenna.
However, in the technique, when a signal transmitted, for example, from a dextro-antenna is reflected by a wall or the like, the polarization of the signal is changed and the signal is converted to a signal transmitted from a levo-antenna, whereby the reception apparatus is not capable of deciding the data accurately. Further, in the technique, it is necessary for the reception apparatus to have two antennas, and thereby it is difficult to miniaturize the reception apparatus.
Furthermore, received data rarely has errors and is received with the high quality when a received electric field strength of the signal is strong, however, received data has errors and the quality of the received signal deteriorates when the received electric field strength of the signal is weak.